1. Field of the Invention
The present invention relates to a hybrid power steering system. More particularly, the present invention relates to a hybrid power steering system that includes two steering assisters for supporting a steering force when a driver operates a steering wheel so as to allow the steering supporting force to be divisionally used according to the increase of speed and the weight of a vehicle, thereby reducing a load applied to any one of the steering assisters such that operating noise can be reduced and endurance can be increased, and allow the use of a motor of a lower power specification even in a case of a truck or a bus that requires a relatively high steering force as compared to a passenger vehicle, so as to reduce production costs and obtain a sufficient steering supporting force, thereby improving the driver's convenience, enhancing fuel efficiency while increasing energy efficiency, and safely maintaining the steering even when one of the steering assisters is not operating normally while the vehicle is being driven.
2. Description of the Prior Art
As generally known in the art, a vehicle steering system is an apparatus that allows a driver to intentionally change the proceeding direction of the vehicle, in which the steering system supports the driver in such a manner that the driver optionally may change the center of rotation, about which the front wheels of the vehicle are turned, so as to cause the vehicle to proceed in a direction desired by the driver.
Meanwhile, when the driver operates the steering wheel of the vehicle, the power steering system supports the steering wheel operating force of the driver using a servo apparatus such that the driver can readily change the proceeding direction of the vehicle with a smaller force.
Such a power steering system is generally classified into an electric power steering (EPS) system and a hydraulic power steering (HPS) system.
The hydraulic power steering system is a steering system configured such that when an oil pump connected to a rotating shaft of an engine supplies hydraulic oil to an actuating cylinder connected with a rack bar, the piston of the actuating cylinder that is supplied with the hydraulic oil is moved to support the steering operation force, thereby allowing the driver to perform a steering operation with a small force.
Meanwhile, the electric power steering system is a steering system that includes a motor in place of the oil pump and the actuating cylinder to support the operating force of the steering wheel with the force of the motor.
FIG. 1 is a schematic view of a conventional electric power steering system.
As illustrated in FIG. 1, the electric power steering system includes a steering system 100 extending from a steering wheel 101 to a pair of opposite vehicle wheels 108, and an auxiliary power mechanism that supplies a steering auxiliary power to the steering system 100.
The steering system 100 includes a steering shaft 102, one end of which is connected to the steering wheel 101 to be rotated together with the steering wheel 101, and the other end of which is connected to a pinion shaft 104 via a pair of universal joints 103. In addition, the pinion shaft 104 is connected to a rack bar through a rack-and-pinion mechanism 105, and the opposite ends of the rack bar is connected to each of the vehicle wheels 108 through a tie rod 106 and a knuckle arm 107. The rack-and-pinion mechanism 105 is configured by a pinion gear 111 formed on a pinion shaft 104, and a rack gear 112 formed on a side of a circumferential surface of a rack bar in which the pinion gear 111 and the rack gear 112 are engaged with each other. Therefore, when the driver operates the steering wheel 101, a torque is produced in the steering system 100, and the vehicle wheels 108 are steered by the torque transmitted through the rack-and-pinion mechanism 105 and the tie rods 106.
The auxiliary power mechanism includes a motor 130 that generates an auxiliary power, and a reducer 140 configured by a worm wheel and a worm to transmit the auxiliary power generated by the motor to the steering shaft 102.
FIG. 2 is a perspective view illustrating a conventional hydraulic power steering system.
As illustrated in FIG. 2, the conventional hydraulic power steering system 200 includes: pinion valve assembly 220 installed at a lower end of the steering shaft equipped with the steering wheel 101 (see FIG. 1) to control the flow direction of the hydraulic oil at the time of steering; and an actuating cylinder 240 configured to apply an acting force to the tie rods 106 that steer the vehicle wheels 108 for steering according to the operation of a pinion valve assembly 220.
The hydraulic power steering system 200 has a hydraulic pressure system in which when the hydraulic oil supplied from the oil pump 230 is supplied to the pinion valve assembly 220 through a pressure hose 235, and the hydraulic oil is selectively supplied to left and right pressure chambers of the actuating cylinder 240 depending on the rotating direction of the steering shaft at the time of steering, the rack bar is slid left or right by the hydraulic pressure to support the steering operating force, and the hydraulic oil, which has completed the action in the pinion valve assembly 220, is returned to an oil storage tank 233 and supplied again to the oil pump 230 through a suction hose 237.
The conventional electric power steering system has problems in that when the errors in manufacturing and assembling in the coupling portions of the worm shaft and the motor shaft accumulate, a non-smooth feeling of a bearing is caused at the time of a large output of power, and the rotational resistance of the worm shaft is increased, in that when the endurance of the worm and the worm wheel is consumed, a gap is formed and a rattle noise is produced due to the backlash, in that when the driver operates the steering handle, the driver experiences an unpleasant feeling due to the impact transferred from a road surface through the vehicle wheels and the steering shafts, and the rattle noise, and in that a high power motor intensifies the operating noise and increases the price.
Also, the hydraulic power steering system has problems in that the hydraulic power steering system increases the manufacturing costs because it is relatively expensive as compared to an electric power steering system, the volume of the hydraulic power steering system is increased in order to provide a required steering supporting force, thereby reducing the fuel efficiency of the vehicle, and in the case of a large output of power, the fluid noise of the hydraulic noise is increased.
In addition, in the situation where the motor or the oil pump stops for any reason, the driver's steering wheel operating force could be abruptly increased, which could cause a fatal safety issue.